Field of Invention
The present invention relates to a method of producing a double-doped scintillation crystal, and more particularly a method of producing a scintillation crystal of rare-earth silicate double-doped with cerium (Ce) and calcium (Ca) or magnesium (Mg). The method especially comprises a thermal annealing process under atmospheric condition, in which the double-doped scintillation crystal obtained from the method has a high yield, less crystal fragmentations, high luminescence intensity and a short decaying time with h a low producing cost.
Description of Related Art
Now, one of widely used scintillation crystal is a rare-earth silicate crystal, especially the cerium-doped lutetium silicate (Ce:Lu2SiO5, LSO). But, after being annealed, most of cerium ions of the crystals are tetravalent Ce4+ due to having no other co-doped ions, leading to reduced luminescence and yellowing crystals.
In the past, the raw material of rare-earth silicate has low purity (mostly 99.95 percents (%)), and thus the raw material itself contains a small amount of impurities like Ca, Mg and other elements. However, concentrations of the above impurities are too low to be quantitatively analyzed, and crystal quality is often unstable. U.S. Pat. No. 7,397,034 disclosed a method of enhancing performance of doped scintillation crystals, in which the colorless and transparent crystals were formed by thermal oxygenation. Nevertheless, yellowing still occurs often, thus it is difficult to control the quality of the crystals. However, the purification technologies of raw materials have been greatly progressed nowadays. The purity of raw material may reach 99.995˜99.999%. Therefore, if the crystal is not doped with other ions, most of the crystals will turn yellow, leading to reduced luminescence and longer decaying time. Currently, the luminescence of doped scintillation crystals may be seven times more than conventional Bismuth Germanate (BGO), and the decaying time can achieve 35˜50 nanoseconds (nsec). Therefore, in view of industrial production, the crystal has to be co-doped with other ions.
As described above, the scintillation crystal of the patent U.S. Pat. No. 7,397,034 was formed by doping lutetium silicate with cerium. Thereafter, the scintillation crystal was cut into pixel samples with specific size, followed by the thermal oxygenation to let oxygen diffuse into the scintillation crystal and make tetravalent cerium (Ce4+) became trivalent cerium (Ce3+). Although the pixel samples of the scintillation crystal have a decaying time of 48 nsec, the obtained pixel samples of the crystal sometimes still turn yellow; and it is hard to control the yield and the oxygen concentration of the pixel samples of the scintillation crystal. Besides, in order to keep the uniformity of luminescence of the crystal, each of the pixel samples has to be subjected to the thermal oxygenation separately. What's more, each of the pixel samples has to be processed to remove the oxide layer formed on the surface of itself during the thermal annealing treatment, so as to increase the luminescence. However, the loss rate of the pixel samples of the crystal may increase during the oxide-layer removing step. As a result, currently conventional methods are obviously not economic for causing a decreasing yield of the pixel samples and a higher producing cost.
Furthermore, if the pixel samples of the crystal of the aforementioned patent are thermally treated in nitrogen atmosphere for excluding oxygen gas, a nitride film will be formed on the crystal surface. In order to remove the nitride film, etching will be required, which increases undesired process steps and is undesirable. Moreover, if the aforementioned patent uses hydrogen gas, mixed oxygen and hydrogen gases may possibly cause undesired explosion, which is still not desirable.
Hence, the prior art does not fulfill all users' requests on actual use. It is necessary to develop a method of producing the double-doped scintillation crystal, in which a yield and luminescence intensity of the double-doped scintillation crystal can be improved.